The chemistry of life, intermolecular interactions

🔹 Abundant Elements in Biomolecules

Q: What are the four most abundant elements in biomolecules? What are their valences?

A:

• Carbon (C): Valence = 4

• Hydrogen (H): Valence = 1

• Oxygen (O): Valence = 2

• Nitrogen (N): Valence = 3

These elements form the backbone of organic molecules.

🔹 Molecular Association Forces

Q: What are the three classes of forces driving molecular association?

A:

1. Covalent bonds (strong, stable)

2. Non-covalent interactions (e.g., ionic, hydrogen bonds)

3. Hydrophobic interactions (entropy-driven)

Q: What three types of interactions contribute to the total energy holding molecules together?

A:

1. Electrostatic (ionic) interactions

2. Hydrogen bonding

3. Van der Waals interactions

🔹 Water Properties

Q: Why is water a dipole?

A: Because the oxygen is more electronegative than hydrogen, pulling electrons toward itself, creating partial charges on both ends.

🔹 Water Properties

Q: How many hydrogen bonds can a water molecule form?

A: Four — 2 as a hydrogen donor, 2 as an acceptor.

🔹 Water Properties

Q: Why does ice float on water?

A: In ice, water molecules form a crystalline structure with more open space, making it less dense than liquid water.

🔹 Water & Electrostatic Forces

Q: How does water affect electrostatic (Coulomb) interactions?

A: Water shields charges due to its high dielectric constant, reducing the strength of ionic interactions compared to vacuum.

🔹 Acid-Base Chemistry

Q: What is the pKa of a carboxylic acid group?

A: Typically around 4.8

🔹 Surface Tension

Q: What is surface tension?

A: The cohesive force at the surface of a liquid caused by molecular interactions.

Q: What is the surface tension of water

A: Approximately 72 mN/m (millinewtons per meter) at room temperature.

🔹 Hydrophobic Effect

Q: What drives the attraction between hydrophobic molecules in water?

A: Entropy — Water forms ordered cages (clathrates) around nonpolar molecules; clustering reduces this ordering and increases entropy.

🔹 Hydrophobic Effect

Q: What experimental parameter measures hydrophobicity?

A: Partition coefficient (log P) — the ratio of a compound’s solubility in octanol vs. water.

🔹 Van der Waals Forces

Q: Are Van der Waals forces long- or short-range?

A: Short-range — they are only effective when molecules are very close together.